1. Field of the Invention
The present invention relates to an ink jet head which ejects ink from an ink ejecting orifice by applying an actuating voltage to a selected inner electrode provided in a piezoelectric layer covering an open plane of an ink pressure chamber formed in a cavity plate.
More particularly, the present invention relates to an ink jet head which utilizes a laminated piezoelectric element constructed of at least two or more piezoelectric layers and polarized in the laminating direction and an outer piezoelectric layer stacked on one plane of the laminated piezoelectric element and polarized in a direction of thickness, the ink jet head being capable of increasing electromechanical transducing efficiency of the laminated piezoelectric element with low actuating voltage and attaining a large deformation of the entire piezoelectric element by a cooperative effect of a shear mode deformation of the laminated piezoelectric element and an expansion mode deformation of the outer piezoelectric layer, and also enhancing rigidity of the ink pressure chamber to reduce loss of the pressure generated therein by applying an actuating voltage to the laminated piezoelectric element to produce an electric field in the laminated piezoelectric element in a perpendicular direction to the polarized direction, whereby the laminated piezoelectric element is deformed in a shear mode and to produce an electric field in the outer piezoelectric layer in a direction parallel to the polarizing direction, whereby the outer piezoelectric layer is deformed in an expansion mode.
2. Description of Related Art
Conventionally, various types of ink jet heads for use in ink jet printers have been proposed. There is, for example, an ink jet head comprising a piezoelectric element fixedly provided on an open plane of an ink pressure chamber formed in a cavity plate, in which when an actuating pulse is applied to an electrode provided in the piezoelectric element, the piezoelectric element is deformed in a shear mode, which causes the change in the volume of the ink pressure chamber to jet an ink drop through an ink jetting orifice.
For example, in U.S. Pat. No. 4,825,227, disclosed is an ink jet head in which an ink pressure chamber is constructed of a chamber plate and a fixing plate, and provided with a single piezoelectric layer on an open plane (upper plane) of the ink pressure chamber. This ink jet head also comprises two electrodes mounted on an upper surface of the piezoelectric layer and at a position corresponding to an edge of each ink pressure chamber and a common electrode mounted on an entire lower surface of the piezoelectric layer. In such the ink jet head, the piezoelectric layer is polarized parallel to the plane of the layer so that the direction of polarization extends along the plane of the piezoelectric layer from the center of each pressure chamber. When an actuating voltage is applied between the two electrodes with opposite polarities, an electric field is generated orthogonal to the direction of polarization of the piezoelectric layer, causing the piezoelectric layer to be deformed in a shear mode to change the volume of the ink pressure chamber. The ink jet head thus jets the ink in the ink pressure chamber via an ink jetting orifice in accordance of the change of the volume of the ink pressure chamber.
Furthermore, in U.S. Pat. No. 4,584,590, disclosed is an ink jet head provided with ink pressure chambers formed in a main plate, a single piezoelectric layer fixedly mounted on an open plane (upper plane) of each ink pressure chamber and electrodes disposed on the opposite surfaces of the piezoelectric layer corresponding to each ink pressure chamber and different electrodes disposed adjacent to an edge away from the ink pressure chamber. In such the ink jet head, the piezoelectric layer is polarized in a direction of thickness of the piezoelectric layer, in which when an actuating voltage is applied to each electrode disposed corresponding to each ink pressure chamber, an electric field is generated in a direction perpendicular to the polarization, thereby causing deformation of the piezoelectric layer in a shear mode. Thus the volume of the ink pressure chamber is changed to jet the ink in the ink pressure chamber via an ink ejecting orifice.
The conventional two ink jet heads, which are different in the polarizing direction of a piezoelectric layer, are common in the following points. One is that application of an actuating voltage between electrodes formed with a space therebetween in the piezoelectric layer causes the generation of an electric field in a direction perpendicular to each polarization direction, thereby causing a shear mode deformation in the piezoelectric layer and changing the volume of the ink pressure chamber, so that ink is jetted via an ink ejecting orifice according to the change in the volume of the ink pressure chamber. Another is that they utilize a single piezoelectric layer.
There is also known an ink jet head utilizing a laminated piezoelectric element constructed of two or more laminated piezoelectric layers. The laminated piezoelectric element used in this type of the ink jet head is a piezoelectric element which is deformed in a so-called expansion mode utilizing a transversal effect mode or a longitudinal effect mode. The ink jet head adopts a structure where a diaphragm and the like is arranged between the piezoelectric element and the ink pressure chamber.
Also in Japanese Patent Application laid-open No. 4-125157, for example, an ink jet head using a laminated piezoelectric element is proposed, in which application of an actuating voltage to the laminated piezoelectric element causes the generation of an electric field in a direction substantially perpendicular to the polarizing direction in the piezoelectric element, thereby deforming the same in a shear mode and changing the volume of a selected ink channel. The ink jet head thus ejects ink according to the change of the volume of the ink channel (see Japanese Patent Application laid-open No. 4-125157 and the like).
However, the above ink jet heads disclosed in U.S. Pat. No. 4,825,227 and No. 4,584,590 are disadvantageous in that, although the direction of the electric field generated upon application of an actuating voltage between electrodes in the piezoelectric layer to jet ink via the ink ejecting orifice is perpendicular to the polarization direction of the piezoelectric layer, the orthogonality between the direction of the electric field and the polarization direction is low due to the use of a single piezoelectric layer. Accordingly, the piezoelectric layer has only a very small electromechanical transducing efficiency upon application of an actuating voltage, so that high actuating voltage is required to increase the electromechanical transducing efficiency of the piezoelectric layer.
The electromechanical transducing efficiency of the piezoelectric layer may be increased if the thickness of the layer is made thin; however, such a thin piezoelectric layer is deteriorated in strength and may be bent in jetting ink in the ink pressure chamber because the piezoelectric layer itself forms one of walls constructing the ink pressure chamber. As a result, there is a problem of causing a deterioration in the pressure generated in the ink pressure chamber.
On the other hand, in the ink jet head disclosed in U.S. Pat. No. 4,825,227, the common electrode formed in the piezoelectric layer on the side toward the ink pressure chamber becomes into direct contact with ink in the ink pressure chamber, there may largely occur corrosion in the common electrode. To avoid the corrosion, an epoxy layer may be provided on the electrode formed in the piezoelectric layer on the side toward the ink pressure chamber so as to cover the electrode from the ink in the ink pressure chamber, as disclosed in aforesaid U.S. Pat. No. 4,584,590. However, the epoxy layer may produce a limitation in deformation of the piezoelectric layer due to the rigidity of the epoxy layer. Movement of the piezoelectric layer is accordingly lowered in jetting ink in accordance with an actuating voltage applied between the electrodes, so that the ink can not be properly jetted via the ink jetting orifice.
The ink jet head utilizing a laminated piezoelectric element, in which the laminated piezoelectric element is deformed in an expansion mode, needs a diaphragm for transmitting the expansive deformation of the laminated piezoelectric element to the ink pressure chamber thereby to jet ink therefrom, resulting in high cost. To achieve a multinozzle head having high resolution, furthermore, two ways are conceivable in case of the use of a laminated piezoelectric element of an expansion mode, namely, one is to arrange a plurality of the piezoelectric elements at a micro pitch and another is to apply recessing process to a single piezoelectric element at a micro pitch. Any ways have a limitation in manufacturing a structure at a micro pitch, and are not suitable for a head with high resolution.
In the ink jet head for jetting ink by deforming a laminated piezoelectric element in a shear mode, the laminated piezoelectric element is constructed of a plurality of piezoelectric layers, so that it has a good electromechanical transducing efficiency and does not need high actuating voltage. However, the laminated piezoelectric element is deformed only in a shear mode, which is not still sufficient in view of the transducing efficiency of the laminated piezoelectric element.